The present invention relates to an analog switch circuit which is used as a recording switch in a tape recorder and, more particularly, to a switch circuit of the type in which a signal with high peak values is applied to the input of the switch circuit when said circuit is in an OFF state.
A typical example of the kind of switch employed in this type of switch circuit is the recording switch of a tape recorder. The switch circuit using such a switch requires a high breakdown withstand voltage against the positive and negative peak values of the input voltage signal applied to the circuit in an OFF state.
FIG. 1 schematically illustrates circuitry containing a record switch circuit and a playback switch circuit. In the figure, reference numeral 11 designates a magnetic head; and, 13, a playback preamplifier. Reference numeral 14 designates a biasing oscillator of the magnetic head 11; SW1 and SW2, switch circuits; C1 to C3, coupling capacitors; and, R1, a resistor.
The operation of the circuitry thus arranged, in record and playback modes, may be described as follows. In a playback mode, the switch SW1 is ON, while the switch circuit SW2 is OFF. Also, in a playback mode, the biasing oscillator 14 does not operate. An acoustic signal read out by the magnetic head 11 is amplified by the preamplifier 13 and sent to a speaker (not shown) connected to the output of the amplifier 13. The read out signal in this mode is very small and its positive and negative peak values are several mV, at most. Therefore, a large breakdown withstand voltage is not required for the switch SW2. In a record mode, the switch circuit SW1 is OFF, while the switch circuit SW2 is ON. A record signal applied from a microphone (not shown) through the recording amplifier 12 is superposed on the output voltage of the biasing oscillator 14, of which the positive and negative peak values are each about 50 V, and then is applied to the magnetic head 11. Accordingly, a maximum of about 100 V of DC voltage is applied to the switch circuit SW1 when it is OFF. In this respect, a breakdown withstand voltage of 100 V or more is required for the switch circuit SW1.
When the switch circuit SW1 is fabricated into a discrete circuit, it is sufficient to use a transistor whose collector-emitter voltage V.sub.CBO is high in an open state. However, when the switch circuit SW1 is fabricated for an integrated circuit, it must have a breakdown withstand voltage high enough to withstand the positive and negative peak values of the applied voltage, with respect to a 0 V level, due to the presence of the diode element, which is parasitically formed. At the present stage of technology in this field, it is difficult to obtain such a high breakdown withstand voltage.
The above problem may now be described in greater detail. When the switch circuit SW1, as shown in FIG. 2, is formed in a discrete circuit, a couple of NPN transistors are connected in series between an input terminal, Tin, and a reference voltage terminal (generally ground,) Tref, with the emitters thereof being interconnected. The switch circuit is controlled by applying control signal S to the bases of the transistors Q1 and Q2. When the circuit as shown in FIG. 3, is fabricated into an integrated circuit, parasitic diodes D1 and D2 are formed between the collector regions of the transistors Q1 and Q2 and the semiconductor substrate Sub. Therefore, to increase the breakdown withstand voltage of the circuit, against the input voltage signal applied to the input terminal Tin, the substrate Sub must be set in an open state, from a potential standpoint. Generally, in the integrated circuit, a plurality of circuits are formed within a single semiconductor chip. Therefore, it is impossible to potentially open the substrate Sub for a specific circuit, in order to prevent an erroneous operation of the remaining circuits. Thus, in the integrated circuits, the substrate Sub is not generally set in an open state. Further, when the switch circuit is fabricated into the integrated circuit, a positive voltage of the AC voltage signal applied to the input terminal Tin is clamped by the parasitic diodes D1 and D2 at a value dropped by a forward voltage drop Vf across the parasitic diodes D1 and D2. The result is a deformation of the waveform of the input voltage signal.
A conventional switch circuit fabricated in the integrated circuit manner, which has succeeded in solving the above problems, is arranged as shown in FIG. 4. As shown, a PNP transistor Q3 is connected between the reference voltage terminal Tref and the input terminal Tin. An NPN transistor Q4 is connected between the base of the transistor Q3 and the substrate Sub. A control signal S is applied to the base of the transistor Q4, to control the ON and OFF states of the switch circuit. This approach reduced the adverse effect caused by the parasitic diode when the switch circuit is fabricated into an integrated circuit. This approach is defective, in that it requires two power sources, positive and negative; since the reference voltage is set at 0 V.